1. Technical Field
The present disclosure relates to a microelectromechanical gyroscope with self-test function and to a method for controlling a microelectromechanical gyroscope.
2. Description of the Related Art
As is known, the use of microelectromechanical systems or MEMS has witnessed an ever-increasing diffusion in various sectors of technology and has yielded encouraging results especially in the production of inertial sensors, microintegrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS of the above type are usually based upon microelectromechanical structures having at least one mass, which is connected to a fixed body (stator) by springs and is movable with respect to the stator according to pre-set degrees of freedom. The movable mass and the stator are capacitively coupled by a plurality of respective comb-fingered and mutually facing electrodes, forming capacitors. The movement of the movable mass with respect to the stator, for example on account of application of an external force, modifies the capacitance of the capacitors; whence it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the applied force. Instead, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force on the movable mass to set it in motion. In addition, in order to obtain electromechanical oscillators, the frequency response of inertial MEMS structures is exploited, which typically is of a second-order low-pass type, with a resonance frequency. By way of example, FIGS. 1 and 2 show the plot of the magnitude and phase of the transfer function between the force applied on the movable mass and its displacement with respect to the stator, in an inertial MEMS structure.
In particular, MEMS gyroscopes have a more complex electromechanical structure, which includes two masses that are movable with respect to the stator and are coupled to one another so as to have a relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the masses is dedicated to driving and is kept in oscillation at the resonance frequency. The other mass is drawn along in oscillating motion and, in the case of rotation of the microstructure with respect to a pre-determined gyroscopic axis with an angular velocity, is subjected to a Coriolis force proportional to the angular velocity itself. In practice, the driven mass operates as an accelerometer that enables detection of the Coriolis force and acceleration and hence makes it possible to trace back to the angular velocity.
As practically any other device, MEMS gyroscopes are subject to defects of fabrication (which can regard both the microstructure and the electronics) and wear, which can diminish the reliability or jeopardize operation thereof completely.
For this reason, gyroscopes, before being installed, are subjected to tests in the factory for proper operation thereof, which enable identification and rejection of defective items.
In many cases, however, it would be important or even vital to be able to carry out sample inspections at any stage of the life of the gyroscope, after installation. In addition to the fact that, in general, it is advantageous to be able to locate components affected by failures in order to proceed to their replacement, MEMS gyroscopes are used also in critical applications, in which a malfunctioning can have disastrous consequences. Just to provide an example, in the automotive field the activation of many air-bag systems is based upon the response supplied by gyroscopes. It is thus evident how important is the function of MEMS gyroscopes with devices capable of carrying out frequent tests on proper operation.
It should moreover be noted that the circuits necessary for the tests must not affect significantly the encumbrance and the level of consumption, which are of ever increasing importance in a large number of applications.